Paper-based chemical assay devices include portable biomedical devices, chemical sensors, diagnostic devices, and other chemical testing devices made of a hydrophilic substrate, such as paper, hydrophobic materials, such as wax or phase change ink, and one or more chemical reagents that can detect chemical assays in test fluids. A common example of such devices includes biochemical testing devices that test fluids such as blood, urine and saliva. The devices are small, lightweight and low cost and have potential applications as diagnostic devices in healthcare, military and homeland security to mention a few. To control the flow of liquids through a porous substrate such as paper, the devices include barriers formed from wax, phase change ink, or another suitable hydrophobic material that penetrates the paper to form fluid channels and other structures that guide the fluid to one or more sites that contain reagents in the chemical assay device.
The current state of the art paper chemical assay devices is limited on fluidic feature resolution and manufacturing compatibility due to uncontrolled reflow of the wax channel after the wax is printed on the paper. The paper and wax are placed in a reflow oven where the wax melts and penetrates into the paper. FIG. 12A and FIG. 12B depict a prior art reflow oven and the spread of melted wax during production of a prior art device. The melted wax, however, tends to spread through the paper in a uniform manner not only through the thickness of the paper but laterally along the surface direction of the paper, which cannot prevent the diffusion of the fluid in the lateral direction, hence difficult to form fine lines, features and other structures. Additionally, while the paper based chemical assay devices are designed to be low-cost devices, the existing manufacturing processes that require separate ovens and adhesives to form multi-layer devices decrease the efficiency of manufacturing these devices and increase the potential for contamination and material compatibility issues. Consequently, improvements to hydrophobic structures within porous substrates and construction of multi-layered chemical assay devices would be beneficial.